In a backbone optical transmission system, it is required to economically accommodate a high-speed client signal and transmit a large amount of information. For realization of such a purpose, a digital coherent transmission scheme that is a combination of coherent detection and digital signal processing has been studied from the viewpoint of improvement of frequency utilization efficiency, and realization of high-speed transmission of a large amount of information through wavelength-division multiplexing transmission using the digital coherent transmission scheme is expected. In the digital coherent transmission scheme, compensation of waveform degradation (for example, chromatic dispersion) generated in a transmission apparatus, a reception apparatus, and an optical fiber transmission path is performed through digital signal processing. Further, in the digital coherent transmission scheme, phase synchronization is established through digital signal processing. For the establishment of phase synchronization, frequency offset estimation (for example, Non-Patent Document 1) and carrier phase synchronization (for example, Non-Patent Document 2) are necessary, and various algorithms have been studied in consideration of the circuit scale and a modulation format.